1. Field of the Invention
The present invention relates to a binder for toner used in flash fixing, a toner, an electrophotographic printing method and an apparatus therefor. More specifically, it relates to a toner exhibiting excellent fixability and void resistance during flashlight irradiation and having reduced fixing odor, an improved binder for the toner, an electrophotographing method using the toner and an apparatus therefor, pertaining to the toner used in the development of an electrostatic image for an electrophotograph and the like.
2. Description of Related Art
Electrophotography has been conventionally known to use such a system as described in U.S. Pat. No. 2,297,691, where a photoconductive insulator (e.g., a photoconductive drum) is commonly used, a uniform electrostatic charge is applied to the photoconductive insulator, for example, by corona discharge, a light image is irradiated on the photoconductive insulator by various means to form an electrostatic latent image, the latent image is developed and visualized using fine particles called toner and after the toner image is transferred, if desired, to paper or the like, the toner image is fixed onto the recording medium such as paper by means of pressurization, heating, exposure to solvent vapor or irradiation by light to obtain a printed matter.
The toner used for developing the electrostatic latent image is conventionally produced by dispersing a coloring agent such as carbon black in a binder resin comprising a natural or synthetic polymer material and finely granulating the obtained dispersion into particles approximately 5 to 20 .mu.m in size. The toner may be used by itself or as a mixture thereof with a carrier such as an iron powder or glass beads in the development of an electrostatic latent image as the toner. In the case where an iron powder or other ferromagnetic powder is used as the carrier, the development is conducted in such a manner that the developing agent consisting of a toner and a carrier is mixed and stirred in a developing apparatus to charge the toner with frictional electrification, a magnet roll in the developing apparatus is rotated to form a magnetic brush, the magnetic brush is transported to the electrostatic latent image portion on the photoconductive sensitized material by rotation of the magnet roll and only the charged toner is adsorbed to the latent image due to the electrical attraction force. After the development, the developing agent which has reduced in toner density is replenished by new toner to maintain the toner density constant and can be repeatedly used.
On the other hand, the toner powder image formed on the photosensitive drum is transferred onto a recording medium (e.g., paper) by corona transfer or roller transfer. The toner powder image transferred to the recording medium is attached to the paper in the state of powder forming an image, where if it is rubbed, for example, by a finger, the powder image is damaged. In order to fix the toner powder image on the recording medium, the powder image must be melted to fix it to the recording medium and various methods are used therefor. Among these methods, the flash fixing method, as a representative example of a photofixing method, is conducted by flash of light from a discharge tube such as a xenon flash lamp and is characterized as follows:
1) due to non-contact fixing, the resolution of a developed image is not deteriorated, PA1 2) the stand-by time after a power source is turned on is not required and a quick start can be realized, PA1 3) even when jamming of the recording paper is caused in the fixing device due to a system failure, ignition is not caused, and PA1 4) fixing is possible irrespective of material or thickness of the recording paper, such as glued paper, preprinted paper or paper of different thickness. PA1 1) due to non-contact fixing, the resolution of a developed image is not deteriorated, PA1 2) the stand-by time after a power source is turned on is not required and a quick start can be realized, PA1 3) even when jamming of the recording paper is caused in the fixing device due to a system failure, ignition is not caused, and PA1 4) printing is possible irrespective of materials or thickness of the recording paper, such as pasted paper, preprint paper or papers different in thickness. Further, since the toner of the present invention is used, excellent flash fixability and superior void resistance can be achieved and thereby a high-quality fixed image can be obtained.
Fixing the toner to the recording paper by flash fixing occurs through the following procedure. As described above, the toner image adheres to the recording paper as a powder when it is transferred to the recording paper and it is readily damaged by rubbing, for example, with a finger. When the flash of light from a discharge tube such as a xenon flash lamp is irradiated thereto, the toner absorbs the energy of flash light, its temperature is raised and thereby the toner is softened and melted to tightly adhere to the recording paper. At the end of the flash of irradiation, the temperature lowers to solidify the image to form a fixed image, thus accomplishing the fixing, and the fixed image firmly adhered to the recording paper is not damaged even when it is rubbed, for example, with a finger.
It is important in flash fixing that the toner is melted and firmly adheres to the recording paper and therefore, the toner must be thoroughly melted by absorbing energy from the flash of light. The total light energy must include not only the energy for melting the toner but also the heat energy diffusing outside and not contributing to the increase of temperature. Accordingly, if the total light energy given is insufficient, the toner cannot be melted thoroughly and as a result, the fixing obtained is not satisfactory. When the light energy is absorbed by the toner, the toner is melted and its viscoelasticity is abruptly lowered.
The physical properties of the melted toner, such as viscoelasticity and surface tension, vary greatly depending on the material and the melting temperature of the binder resin constituting the toner and if the surface tension of the toner at the time of melting and fixing overpowers the viscoelasticity thereof, the toner aggregates and the toner which was uniformly present on the image portion moves to cause an image drop-out phenomenon called a void (aggregation void) on the fixed image, resulting in a reduction in the image density. Further, if an excess amount of energy is given, the toner boils and the melted toner is blown off by the explosive expansion of gas present in spaces in the toner powder image and the gas generated by the toner decomposition, to generate a void (explosion void) in the fixed image to thereby cause a reduction in the image density.
Accordingly, the toner must not be hardly susceptible to the generation of voids due to aggregation or moving of the toner and in this concern, the use of a binder resin having a low surface tension and a high viscoelasticity is needed so that the viscoelasticity of toner overpowers the surface tension during melting.
As known from the foregoing, in order to have good fixability, in the flash fixing system, the total amount of light energy irradiated must be sufficiently large and the toner used or the binder resin constituting the toner must have physical properties of a low melting point and a low melt viscoelasticity so that it swiftly absorbs the light energy of flash light and melts to permeate the recording medium such as paper. On the other hand, in order to prevent voids, in the flash fixing system, excessive energy must not be applied but the energy must be applied to the toner in such a manner that melt properties (e.g., melting temperature) of the toner are controlled and the toner used needs to have a melt viscoelasticity sufficiently high to prevent movement of the toner causing generation of voids.
The toner for flash fixing must also not generate a bad odor even if it is heated to a high temperature during flash fixing.
The main object of the present invention is to provide a toner and a binder for the toner which have excellent fixability and void resistance in flash fixing and have a low fixing odor.